DYE-SENSITIZED SOLAR CELL
A solar cell is a photonic device that converts photons with specific wavelengths to electricity. A dye-sensitized solar cell (DSSC) is a low-cost solar cell belonging to the group of thin film solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system. The modern version of a dye solar cell, also known as the Grätzel cell, was originally co-invented in 1988 by Brian O'Regan and Michael Grätzel. In general, a DSC comprises a nanocrystalline titanium dioxide (TiO2) electrode modified with a dye fabricated on a transparent conducting oxide (TCO), a platinum (Pt) counter electrode, and an electrolyte solution with a dissolved iodide ion/triiodide ion redox couple between the electrodes.
Even though the conversion efficiency of dye-sensitized cells is lower than that of some other thin-film cells, their price to performance ratio is sufficient to make them an important player in the solar market. The advantages of DSSCs are listed below: *
They are the most efficient third-generation solar technology available, absorbing more sunlight per surface area than standard silicon-based solar panels. *
DSSCs are an attractive replacement for current technologies in low density applications such as rooftop solar collectors, where the light weight and mechanical robustness of the printable cell is a key benefit. *
These may not be as attractive for large-scale deployments where high-efficiency, high-cost cells are more suitable. However, even minimal future increases in the conversion efficiency of the DSSC may make it suitable for some of these applications. *
DSSCs work even in low-light conditions such as non-direct sunlight and cloudy skies. *
They are economical, easy to manufacture and constructed from abundant and stable resource materials. *
The mechanical robustness of the DSSC leads indirectly to higher efficiencies at a range of temperatures. *
Normally, DSSCs are built with just a thin conductive plastic top layer, helping heat to be radiated away more easily and hence operate at low internal temperatures. Construction
In the case of the original Grätzel and O'Regan design, the cell has 3 primary parts. On top is a transparent anode made of fluoride-doped tin dioxide (SnO2:F) deposited on the back of a (typically glass) plate. On the back of this conductive plate is a thin layer of titanium dioxide (TiO2), which forms into a highly porous structure with an extremely high surface area. TiO2 only absorbs a small fraction of the solar photons (those in the UV). The plate is then immersed in a mixture of a photosensitive ruthenium-polypyridine dye (also called molecular sensitizers) and a solvent. After soaking the film in the dye solution, a thin layer of the dye is left covalently bonded to the surface of the TiO2. A separate plate is then made with a thin layer of the iodide electrolyte spread over a conductive sheet, typically platinum metal. The two plates are then joined and sealed together to prevent the electrolyte from leaking. The construction is simple enough that there are hobby kits available to hand-construct them. Although they use a number of "advanced" materials, these are inexpensive compared to the silicon needed for normal cells because they require no expensive manufacturing steps. TiO2, for instance, is already widely used as a paint base. Operation
Sunlight enters the cell through the transparent SnO2:F top contact, striking the dye on the surface of the TiO2. Photons striking the dye with enough energy to be absorbed create an excited state of the dye, from which an electron can be "injected" directly into the conduction band of the TiO2. From there it moves by diffusion (as a result of an electron concentration gradient) to the clear anode on top. Meanwhile, the dye molecule has lost an electron and the molecule will decompose if another electron is not...
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